© 2018 Kansai University. All rights reserved. Water circulation induced by a standard Taiwanese paddle-wheel aerator in a rectangular reservoir with the dimensions (L, W, D) 50 × 19 × 1.3 m3 was studied to evaluate of the efficiency of the aeration process in aquaculture. The water velocity was measured using an electromagnetic water current meter located at 12 grid points selected downstream of the aerator. We found that the water velocity field is unidirectional. This suggests that fluid mixing in the vertical direction may not be comparable to what would be expected from the intense splashes and loud noise produced by the operating aerator.